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This will be used in the next patch. Signed-off-by: Ackerley Tng Reviewed-by: David Hildenbrand (Arm) --- tools/testing/selftests/kvm/include/kvm_syscalls.h | 1 + 1 file changed, 1 insertion(+) diff --git a/tools/testing/selftests/kvm/include/kvm_syscalls.h b/tools/tes= ting/selftests/kvm/include/kvm_syscalls.h index d4e613162bba9..843c9904c46f6 100644 --- a/tools/testing/selftests/kvm/include/kvm_syscalls.h +++ b/tools/testing/selftests/kvm/include/kvm_syscalls.h @@ -77,5 +77,6 @@ __KVM_SYSCALL_DEFINE(munmap, 2, void *, mem, size_t, size= ); __KVM_SYSCALL_DEFINE(close, 1, int, fd); __KVM_SYSCALL_DEFINE(fallocate, 4, int, fd, int, mode, loff_t, offset, lof= f_t, len); __KVM_SYSCALL_DEFINE(ftruncate, 2, unsigned int, fd, off_t, length); +__KVM_SYSCALL_DEFINE(madvise, 3, void *, addr, size_t, length, int, advice= ); =20 #endif /* SELFTEST_KVM_SYSCALLS_H */ --=20 2.53.0.345.g96ddfc5eaa-goog From nobody Fri Apr 17 10:34:52 2026 Received: from mail-pf1-f201.google.com (mail-pf1-f201.google.com [209.85.210.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4588E316904 for ; 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Fri, 20 Feb 2026 15:54:45 -0800 (PST) Date: Fri, 20 Feb 2026 23:54:36 +0000 In-Reply-To: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: X-Mailer: git-send-email 2.53.0.345.g96ddfc5eaa-goog Message-ID: <8048d04f150326d1e2231318aa9f1b3fce3e2e2c.1771630983.git.ackerleytng@google.com> Subject: [PATCH v2 2/2] KVM: selftests: Test MADV_COLLAPSE on guest_memfd From: Ackerley Tng To: kartikey406@gmail.com, seanjc@google.com, pbonzini@redhat.com, shuah@kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org Cc: vannapurve@google.com, Liam.Howlett@oracle.com, ackerleytng@google.com, akpm@linux-foundation.org, baohua@kernel.org, baolin.wang@linux.alibaba.com, david@kernel.org, dev.jain@arm.com, i@maskray.me, lance.yang@linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, npache@redhat.com, ryan.roberts@arm.com, shy828301@gmail.com, stable@vger.kernel.org, syzbot+33a04338019ac7e43a44@syzkaller.appspotmail.com, ziy@nvidia.com Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" guest_memfd only supports PAGE_SIZE pages, and khugepaged or MADV_COLLAPSE collapsing pages may result in private memory regions being mapped into host page tables. Add test to verify that MADV_COLLAPSE fails on guest_memfd folios, and any subsequent usage of guest_memfd memory faults in PAGE_SIZE folios. Running this test should not result in any memory failure logs or kernel WARNings. This selftest was added as a result of a syzbot-reported issue where khugepaged operating on guest_memfd memory with MADV_HUGEPAGE caused the collapse of folios, which then subsequently resulted in a WARNing. Link: https://syzkaller.appspot.com/bug?extid=3D33a04338019ac7e43a44 Suggested-by: David Hildenbrand Signed-off-by: Ackerley Tng --- .../testing/selftests/kvm/guest_memfd_test.c | 70 ++++++++++++++++++- 1 file changed, 67 insertions(+), 3 deletions(-) diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing= /selftests/kvm/guest_memfd_test.c index 618c937f3c90f..0edbc7cf6c1ad 100644 --- a/tools/testing/selftests/kvm/guest_memfd_test.c +++ b/tools/testing/selftests/kvm/guest_memfd_test.c @@ -171,6 +171,64 @@ static void test_numa_allocation(int fd, size_t total_= size) kvm_munmap(mem, total_size); } =20 +static void test_collapse(int fd, uint64_t flags) +{ + const size_t pmd_size =3D get_trans_hugepagesz(); + void *reserved_addr; + void *aligned_addr; + char *mem; + off_t i; + + /* + * To even reach the point where the guest_memfd folios will + * get collapsed, both the userspace address and the offset + * within the guest_memfd have to be aligned to pmd_size. + * + * To achieve that alignment, reserve virtual address space + * with regular mmap, then use MAP_FIXED to allocate memory + * from a pmd_size-aligned offset (0) at a known, available + * virtual address. + */ + reserved_addr =3D kvm_mmap(pmd_size * 2, PROT_NONE, + MAP_PRIVATE | MAP_ANONYMOUS, -1); + aligned_addr =3D align_ptr_up(reserved_addr, pmd_size); + + mem =3D mmap(aligned_addr, pmd_size, PROT_READ | PROT_WRITE, + MAP_FIXED | MAP_SHARED, fd, 0); + TEST_ASSERT(IS_ALIGNED((u64)mem, pmd_size), + "Userspace address must be aligned to PMD size."); + + /* + * Use reads to populate page table to avoid setting dirty + * flag on page. + */ + for (i =3D 0; i < pmd_size; i +=3D getpagesize()) + READ_ONCE(mem[i]); + + /* + * Advising the use of huge pages in guest_memfd should be + * fine... + */ + kvm_madvise(mem, pmd_size, MADV_HUGEPAGE); + + /* + * ... but collapsing folios must not be supported to avoid + * mapping beyond shared ranges into host userspace page + * tables. + */ + TEST_ASSERT_EQ(madvise(mem, pmd_size, MADV_COLLAPSE), -1); + TEST_ASSERT_EQ(errno, EINVAL); + + /* + * Removing from host page tables and re-faulting should be + * fine; should not end up faulting in a collapsed/huge folio. + */ + kvm_madvise(mem, pmd_size, MADV_DONTNEED); + READ_ONCE(mem[0]); + + kvm_munmap(reserved_addr, pmd_size * 2); +} + static void test_fault_sigbus(int fd, size_t accessible_size, size_t map_s= ize) { const char val =3D 0xaa; @@ -350,14 +408,17 @@ static void test_guest_memfd_flags(struct kvm_vm *vm) } } =20 -#define gmem_test(__test, __vm, __flags) \ +#define __gmem_test(__test, __vm, __flags, __gmem_size) \ do { \ - int fd =3D vm_create_guest_memfd(__vm, page_size * 4, __flags); \ + int fd =3D vm_create_guest_memfd(__vm, __gmem_size, __flags); \ \ - test_##__test(fd, page_size * 4); \ + test_##__test(fd, __gmem_size); \ close(fd); \ } while (0) =20 +#define gmem_test(__test, __vm, __flags) \ + __gmem_test(__test, __vm, __flags, page_size * 4) + static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags) { test_create_guest_memfd_multiple(vm); @@ -367,9 +428,12 @@ static void __test_guest_memfd(struct kvm_vm *vm, uint= 64_t flags) =20 if (flags & GUEST_MEMFD_FLAG_MMAP) { if (flags & GUEST_MEMFD_FLAG_INIT_SHARED) { + size_t pmd_size =3D get_trans_hugepagesz(); + gmem_test(mmap_supported, vm, flags); gmem_test(fault_overflow, vm, flags); gmem_test(numa_allocation, vm, flags); + __gmem_test(collapse, vm, flags, pmd_size); } else { gmem_test(fault_private, vm, flags); } --=20 2.53.0.345.g96ddfc5eaa-goog